Nitrate aerosol: implications for global air quality, human health and climate

This thesis investigates the influence of atmospheric nitrate aerosol on air quality, human health and the present day aerosol cloud-albedo radiative effect. This research represents the first application of the UM-UKCA model with the nitrate-extended version of the GLOMAP-mode aerosol microphysics module. The UM-UKCA model generally represents the spatial distribution of annual mean nitrate, ammonium and sulphate aerosol concentrations well (R > 0.6) over Europe and North America but performs less well over East Asia. The model tends to underestimate concentrations relative to ground-based and aircraft observations. However, global burdens and mean surface concentrations of nitrate (NO3-), ammonium (NH4+) and sulphate (SO42-) aerosol are within the multi-model range simulated by participating models in the AeroCom Phase III (AP3) nitrate intercomparison. The inclusion of heterogeneous N2O5 hydrolysis in UM-UKCA decreases regional low model bias of nitrate aerosol, particularly in wintertime. The size distribution of simulated NO3- is sensitive to the approach applied to treat the gas-particle partitioning of the inorganic NO3--NH4+-SO42--Na+-Cl- aerosol system. Reverting to an equilibrium approach to treat the gas-particle partitioning leads to overestimation of coarse mode nitrate aerosol, relative to applying a hybrid approach, at the expense of nitric acid uptake into the fine mode. The contribution of nitrate aerosol to surface PM2.5 and to the PM2.5-attributable health burden over the EU28, USA, India and China is quantified. Of these regions, nitrate aerosol made the greatest relative contribution to present day (year 2008) annual mean AWM and PWM PM2.5 (13.4% - 16.3%) and to PM2.5-attributable mortality (15.6%; 31,500 [20,100 - 41,300] deaths) over the EU28. Comparison to a scenario with year 1980 emissions, demography and background disease rates showed that the relative contribution of nitrate aerosol to the total ambient PM2.5-attributable mortality increased over the EU28 (6.2% to 15.6%), USA (3.9% to 7.8%), India (1.3% to 3.4%) and China (4.0% to 5.8%) as a result of recent decadal changes. Finally, it is found that simulated N3, N50 and CDNC undergo large relative regional increases of up to 60%, when nitrate aerosol formation is included using a hybrid gas-particle partitioning approach. A global mean PD nitrate cloud albedo radiative effect of -0.007 Wm-2 was calculated using the hybrid approach, though the influence of nitrate aerosol on regional radiative effects was larger (-0.5 to +1 W m-2). Reverting to an equilibrium gas-particle partitioning approach resulted in a weaker response of aerosol number concentrations and CDNC to nitrate aerosol formation over much of the Northern Hemisphere and cloudy marine regions in the Southern Hemisphere, leading to a sign reversal in the PD nitrate cloud albedo radiative effect to +0.011 W m-2. It is therefore concluded that models applying an equilibrium approach to treat nitrate aerosol formation may under-represent the influence of nitrate aerosol on aerosol properties and aerosol-cloud radiative effects.